Certain portable electronic devices, such as radiotelephones, must be visible in both low-light and bright-light environments. A standard emissive display uses a pixelated, or patterned, light source in which different regions of the display can be selectively turned on to emit light. Regions of the display that are turned off do not emit light; however, in bright light situations, the “off” portion of the display can be as bright or brighter than the “on” portion of the display due to the highly reflective nature of the light-emitting materials (generally phosphors). A contrast filter, also called a neutral density filter, can be placed over the display and uses a partially light-absorbing film to absorb some of the emitted and reflected light from the display. The filter, however, reduces the brightness of the display in all viewing conditions.
Polymer-dispersed liquid crystal (PDLC) display technology is a low-cost, all-plastic display technology using micron-sized liquid crystal droplets dispersed in a solid polymer matrix. Dichroic dye molecules added to the liquid crystal droplets cause a PDLC display to exhibit the color of the dye when no electric field is applied to the display. In this colored state, the dye molecules and liquid crystal droplets are randomly arranged and the display is opaque. This is conceptually a “closed optical shutter” condition. When an electric field of predetermined strength and orientation is applied, the dye molecules and the liquid crystal droplets reorient to allow light to pass through the PDLC layer. This is conceptually an “open optical shutter” condition. In this transparent state, the display reveals the color of the display backing.
If the display backing is a reflective surface, in bright-light environments, ambient light makes two passes through the PDLC layer where the PDLC is in an open shutter condition. Optimizing the display for bright-light environments requires using a PDLC dye molecule concentration where the closed shutter state is dark yet without excess residual absorption in the open shutter state that would affect the display brightness. In low-light environments, a PDLC display can be backlit. When the display is backlit, however, the backlight only makes one pass through the PDLC display. When the PDLC dye molecule concentration is optimized for bright-light conditions, the backlit display has significantly degraded contrast and the “off” pixels can appear to be turned on.
Thus there is a need for a low-cost, low-current drain display that exhibits high contrast in both bright-light and low-light conditions.